Method and device for evaporating a fluid

ABSTRACT

A method and device for evaporating a predeterminable volume of fluid includes successive addition of partial volumes of the predeterminable volume to a supply line at different adding rates, at least partially evaporating the partial volumes forming vapor film between them and a supply line wall, conveying the partial volumes through the supply line to an evaporator surface, and applying the partial volumes to an evaporator surface region varying as a function of mass and/or volume adding rate of the partial volume, permitting effective evaporation of fluid, particularly urea/water solution. Utilization of the highest possible proportion of evaporator surfaces is achieved by mass and/or volume addition rate-dependent distribution of impingement surfaces on the evaporator surface. This heating strategy in the supply line region ensures the Leidenfrost effect when individual partial volumes are added. As even a distribution as possible is achieved using a corresponding geometrical configuration of the evaporator channel.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation, under 35 U.S.C. §120, of copending InternationalApplication No. PCT/EP2008/056233, filed May 21, 2008, which designatedthe United States; this application also claims the priority, under 35U.S.C. §119, of German Patent Application DE 10 2007 024 081.5, filedMay 22, 2007; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method and a device for evaporating afluid. In particular, the fluid is a urea/water solution which is usedas a reducing agent precursor of the reducing agent ammonia in theselective catalytic reduction of nitrogen oxides in the exhaust gassystem of internal combustion engines.

In the use of selective catalytic reduction (SCR) for reducing thenitrogen oxide content in the exhaust gas of internal combustionengines, in particular in automobiles, ammonia is often used as areducing agent acting selectively on nitrogen oxides. In particular inmobile applications, ammonia is generated from urea by thermolysisand/or hydrolysis. Urea is often stored in the form of a urea/watersolution and then, as required, either injected in solution form intothe exhaust gas system or evaporated outside the exhaust gas and thensupplied to the exhaust gas as an ammonia or urea-containing vapor. Inthe latter case, specifically the evaporation of relatively largevolumes in cases of dynamic loads is often a problem, since theapplication of the evaporation enthalpy to the evaporator drains quite alot of energy, so that the evaporator cools and the cooling may be sosevere that the fluid is no longer fully evaporated.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method and adevice for evaporating a fluid, which overcome the hereinafore-mentioneddisadvantages and at least some of the drawbacks of the heretofore-knownmethods and devices of this general type and in which even relativelylarge volumes of the fluid can be fully evaporated.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for evaporating apredeterminable volume of a fluid. The method comprises:

-   -   a) successively adding at least one first partial volume of the        predeterminable volume into a feed line at a first volume adding        rate, and successively adding at least one second partial volume        of the predeterminable volume, being different from the first        partial volume, into the feed line at a second volume adding        rate;    -   b) at least partially evaporating the partial volumes to form a        vapor film between the respective partial volume and a wall of        the feed line;    -   c) guiding the partial volumes through the feed line to an        evaporator surface; and    -   d) applying the partial volumes to a region of the evaporator        surface disposed at a position varying as a function of at least        one of the following parameters:        -   i) a mass of the corresponding partial volume, or        -   ii) a volume adding rate of the corresponding partial            volume.

Step a) therefore includes in particular dividing the predeterminablevolume into a plurality of partial volumes of different size. This canbe carried out by a corresponding control device, so that, prior toaddition, a plurality of partial volumes of the predeterminable volumeare calculated and the partial volumes are then added into the feedline, for example by way of a correspondingly embodied metering pump.Step a) also includes the case in which the fluid is conveyedcontinuously into the feed line, with the volume flow being variableover time, so that for this reason first and second partial volumes aresuccessively added into the feed line. In this case, the volume addingrate is varied over time, for example through corresponding variation ofthe conveying power of a pump and/or through corresponding restrictors,such as for example valves, which can be used to vary the volume addingrate for opening or closing in a time-controlled manner. The amplitudeof the volume adding rate can be varied continuously and/ordiscontinuously. The amplitude of the volume adding rate can remainconstant over a predeterminable time period and the length of this timeperiod is preferably variable.

As a result of step d), different regions of the evaporator surface areacted on by the partial volumes of different size as a function of themass and/or volume adding rate. This leads to more uniform utilizationof the evaporator surface and thus to more uniform and not justpoint-by-point cooling of the evaporator surface. This results in muchmore complete evaporation than in other systems of the prior art, sincethe risk of the evaporator surface cooling so intensively as to preventfurther evaporation is greatly reduced.

The fluid is particularly preferably a urea/water solution. It isparticularly preferable in this regard to employ or use the method forsupplying a urea and/or ammonia-containing gas flow to the exhaust gassystem of an internal combustion engine, in particular in mobileapplications such as for example in automobiles and/or motorcycles.

In accordance with another mode of the method of the invention, theevaporator surface includes the surface of at least one evaporatorchannel. Preferably, the feed line and evaporator channel are a commonchannel.

In accordance with a further mode of the method of the invention, it isalso preferable for step d) to be based on inertia effects. Thus, adistribution which is as uniform as possible of the partial volumes onthe evaporator surface can be achieved due to the different masses ofthe individual partial volumes while utilizing inertia effects. This canbe carried out in particular as a result of the fact that the evaporatorchannel is curved, changes its radius of curvature and/or that theevaporator channel changes its through-flow cross section.

In accordance with an added mode of the method of the invention, thefluid includes urea in aqueous solution, if appropriate with furtheradditives, for example of formic acid. Aqueous solutions of urea of thistype are available under the trademarks Ad Blue® or Denoxium®.

With the objects of the invention in view, there is also provided adevice for evaporating a predeterminable volume of a fluid using themethod according to the invention. The device comprises a feed line, anevaporator surface for receiving the fluid through the feed line, anevaporator channel having a surface included in the evaporator surface,and at least one variable parameter selected from the group consistingof:

a) a radius of curvature of the evaporator channel,

b) a through-flow cross section of the evaporator channel, and

c) a volume adding rate of the fluid.

By varying the radius of curvature, the curvature and/or thethrough-flow cross section, it is possible, due to inertia effects, fordifferently sized partial volumes of the predeterminable volume toimpinge at different points of the evaporator surface. Thus, differentregions of the evaporator surface are drawn on for applying theevaporation enthalpy and for further heating of the vapor. This preventspoint-by-point cooling of the evaporator surface during evaporating ofrelatively large predeterminable volumes. The variability of the volumeadding rate of the fluid is achieved by way of an accordinglyactivatable conveying device such as for example pumps and/or valves.

In accordance with another feature of the device of the invention, atleast a part of the evaporator channel is provided with a porous coatingwhich catalyzes in particular the hydrolysis of a reducing agentprecursor to form a reducing agent. Particularly preferably, the feedline and the evaporator channel are made of a material which iscorrosion-resistant upon the addition and evaporation of a urea/watersolution. In particular, the material can include a correspondingspecial steel, titanium and/or aluminum.

In accordance with a concomitant feature of the device of the invention,there is preferably provided a conveying device including at least onepump for conveying the fluid from a reservoir into the feed line.Preference is given to the configuration of the pump as a metering pump,the conveying power of which, and thus the volume adding rate of thefluid, is regulatable. Alternatively or additionally, a conveying pumpcan be embodied as a conveying device through the use of which aconveying power and/or a conveying pressure of the fluid can bepredetermined. By varying the conveying power and/or the conveyingpressure and activating a corresponding valve embodied between thereservoir and feed line, it is possible to vary the volume adding rateof the fluid.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method and a device for evaporating a fluid, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic, cross-sectional view of an exemplaryembodiment of a device according to the invention illustrating a varyingthrough-flow cross section;

FIG. 2 is a view similar to FIG. 1 of an exemplary embodiment of adevice according to the invention illustrating a radius of curvature;

FIG. 3 is an enlarged, fragmentary, cross-sectional view of an exemplaryembodiment of a device according to the invention with a porous coatingon an evaporator surface;

FIG. 4 is a view similar to FIGS. 1 and 2 of a second exemplaryembodiment of a device according to the invention; and

FIGS. 5 to 7 are graphs showing examples of variations of a volumeadding rate of the fluid.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a first exemplaryembodiment of a device 1 according to the invention for evaporating afluid. The device 1 has a channel 2 which may be divided into a feedline 3 and an evaporator channel 4. The feed line 3 serves in this caseto supply the fluid, while the evaporator channel 4 has walls serving asevaporator surfaces 5. In this case, the device 1 has a solid body whichis made of aluminum and in which the channel 2 is formed, for example bymilling. The device 1 also has a non-illustrated electrical heatingdevice which can be supplied with electric current through terminals 6.The device 1 can be electrically heated in this way.

The corresponding fluid can be conveyed from a reservoir 8, through ametering pump 7, into the feed line 3. The conveying power of themetering pump 7, and thus the volume adding rate of the fluid into thefeed line 3, is variable. According to the invention, a predeterminablevolume of the fluid, in this case a urea/water solution for providingammonia as a reducing agent in the exhaust gas of internal combustionengines, is to be evaporated. According to the invention, thepredeterminable volume, the amount of which may be calculated forexample from the nitrogen oxide concentration present in the exhaustgas, is divided in this case into individual partial volumes ofdifferent size. Thus, it is for example, possible for there to be formeda first partial volume corresponding to one third of the predeterminablevolume and a second partial volume corresponding to two thirds of thepredeterminable volume. Other divisions are possible, for example into afirst partial volume with one sixth of the predeterminable volume, asecond partial volume with two sixths of the predeterminable volume anda third partial volume corresponding to half of the predeterminablevolume. In this case, the metering pump 7 can be utilized in anadvantageous manner for conveying the partial volumes into the feed line3 and then into the evaporator channel 4. Alternatively, the volumeadding rate of the fluid can be varied, by activating the metering pump7 accordingly, in such a way that the predeterminable volume isevaporated. In this case, it is also possible in an advantageous mannerfor the running time of the exhaust gas from the internal combustionengine up to the reaction with ammonia to be utilized and the meteringpump 7 to be controlled accordingly.

The corresponding partial volumes are partially evaporated in theevaporator channel 4 or the feed line 3, so that a vapor film is formedbetween the partial volume and a wall of the feed line 3. This is based,in particular, on the so-called Leidenfrost effect. In this case, thewalls of the feed line 3 are heated in such a way that this Leidenfrosteffect occurs in a targeted manner. This produces a vapor film betweenthe partial volume and the wall of the feed line 3. Due to the pressuregradient maintained by the metering pump 7, this partial volume isguided through the feed line 3 and the evaporator channel 4. In thiscase, the individual partial volumes are guided toward the evaporatorsurface 5. In the exemplary embodiment, this evaporator surface 5 is awall of the evaporator channel 4. Changing the curvature and/or thecross section of the evaporator channel 4 which can be flowed throughallows the partial volumes to impinge on the evaporator surface 5 as afunction of the mass of the individual partial volumes. The fact thatdifferent partial volumes are added allows not only a partial region ofthe evaporator surface 5 to be utilized for evaporating the fluid, butrather a scattering of the evaporator surfaces 5 being used is alsoachieved. Further scattering is alternatively or additionally achievedby varying the volume adding rate of the fluid. The scattering of theevaporator surfaces 5 being used advantageously leads to the evaporatorsurface 5 being effectively utilized and, in particular duringevaporation of relatively large predeterminable volumes of the fluid,the evaporator surface 5 is utilized uniformly. This leads to asignificantly improved evaporation result since, in the case ofsubstantially point-by-point utilization of the evaporator surface 5 andrelatively large predeterminable volumes, the loss of heat within thedevice 1 is so great that it is not possible to ensure completeevaporation of the predeterminable volume. Furthermore, the temperatureof the feed line 3 and/or of the evaporator channel 4 can be regulatedaccordingly to achieve as complete as possible evaporation of the fluid.

Thus, the procedure according to the invention advantageously preventsincomplete evaporation in which drops of the fluid issue unevaporatedfrom the evaporator channel 4. The application of the individual partialvolumes to a region of the evaporator surface 5, the position of whichis varied as a function of the mass and/or the volume adding rate of thefluid of the corresponding partial volume, can be achieved by way of aplurality of measures. For example, the curvature and/or the crosssection of the evaporator channel 4 to be flowed through in this casecan be varied. Generally, use is made in this case of the inertiaeffects leading to different impingement regions on the evaporatorsurface 5, due to the different mass of the various partial volumes.

FIG. 1 shows in this case a first through-flow cross section 9 and asecond through-flow cross section 10 of the evaporator channel 4. Thefirst through-flow cross section 9 is larger than the secondthrough-flow cross section 10. A vapor flow 13 of the evaporated fluidleaves the device 1.

FIG. 2 shows that the curvature of the evaporator channel 4 changes. Afirst circle of curvature 11 and a second circle of curvature 12 areindicated for this purpose. As may be seen, the radius of curvaturechanges in this case, leading to a mass-dependent deposition of thepartial volumes on the evaporator surface 5 and thus to locally resolvedevaporation of the individual partial volumes.

FIG. 3 shows a portion of an exemplary embodiment in which theevaporator surface 5 has a porous coating 14 which catalyzes thehydrolysis of urea to form ammonia.

FIG. 4 shows an alternative exemplary embodiment of a device 1 accordingto the invention. Instead of a metering pump 7, the present exemplaryembodiment has a conveying pump 15 through the use of which theurea/water solution is conveyed out of the reservoir 8. The addition ofthe urea/water solution to the feed line 3 is regulated through aregulatable ⅔-way valve 16. The urea/water solution can be guided eitherinto the feed line 3 or back into the reservoir 8 through this valve 16.The volume adding rate of the fluid into the feed line 3 can be variedby way of the duration of the opening of the valve 16 up to the feedline 3, and also the amount which passes through the valve 16 into thefeed line 3 and can be regulated by way of regulating the amountconveyed by the conveying pump 15.

FIG. 5 shows a first example of a course of a volume adding rate 17 intothe feed line 3 over time. This example can be achieved by way of thecorresponding use of a conveying pump 15 and a valve 16, wherein thevalve 16 can have different free through-flow cross sections. In thiscase, the amount of fluid entering the feed line 3 through the valve 16is therefore regulatable per unit of time. The volume adding rate 17 canbe controlled by varying the time period in which the valve 16 is openedor closed to the feed line 3 and by varying the amount of fluid flowingthrough the valve 16. The volume adding rate 17 can be variedaccordingly, as shown in FIGS. 6 and 7, by way of a correspondingactivation of the metering pump 7 and/or the conveying pump 15 as wellas of the valve 16. In FIGS. 5 to 7, both time t and a volume addingrate V/t are indicated in arbitrary units.

The method according to the invention and the device 1 according to theinvention advantageously allow evaporation of a fluid, in particular ofa urea/water solution, that is as effective as possible. The utilizationof the largest possible proportion of the evaporator surface 5 isachieved as a result of the mass and/or volume adding rate-dependentdistribution or division of the impingement surfaces on the evaporatorsurface 5. For this purpose, a heating strategy is operated in anadvantageous manner in the region of the feed line 3, which ensures thatthe Leidenfrost effect occurs each time the individual partial volumesare added. In this manner, a distribution which is as uniform aspossible can be achieved by way of a corresponding configuration of thegeometry of the evaporator channel 4.

1. A method for evaporating a predeterminable volume of a fluid, themethod comprising the following steps: providing a urea and watersolution as the fluid for providing ammonia as a reducing agent inexhaust gas of internal combustion engines; calculating thepredeterminable volume of the fluid from a nitrogen oxide concentrationpresent in the exhaust gas; dividing the predeterminable volume of thefluid into individual partial volumes of different size; a) successivelyadding at least one first partial volume of the predeterminable volumeof the fluid into a feed line at a first volume adding rate, andsuccessively adding at least one second partial volume of thepredeterminable volume of the fluid, being different from the at leastone first partial volume, into the feed line at a second volume addingrate; b) at least partially evaporating the partial volumes to form avapor film between the at least one first partial volume and a wall ofthe feed line; c) guiding the partial volumes through the feed line toan evaporator surface; and d) applying the partial volumes to a regionof the evaporator surface disposed at a position varying as a functionof at least one of the following parameters: i) a mass of thecorresponding partial volume, or ii) a volume adding rate of thecorresponding partial volume.
 2. The method according to claim 1,wherein the evaporator surface includes a surface of at least oneevaporator channel.
 3. The method according to claim 1, which furthercomprises carrying out step d) based on inertia effects.
 4. The methodaccording to claim 2, wherein the at least one evaporator channel iscurved and changes its radius of curvature.
 5. The method according toclaim 2, wherein the at least one evaporator channel changes itsthrough-flow cross section.